Fix object property value in mlx_lm.server chat completions response to match OpenAI spec (#1119)

These were "chat.completions" and "chat.completions.chunk"
but should be "chat.completion" and "chat.completion.chunk"
for compatibility with clients expecting an OpenAI API.

In particular, this solves a problem in which aider 0.64.1 reports
hitting a token limit on any completion request, no matter how small,
despite apparently correct counts in the usage property.

Refer to:

https://platform.openai.com/docs/api-reference/chat/object

> object string
> The object type, which is always chat.completion.

https://platform.openai.com/docs/api-reference/chat/streaming

> object string
> The object type, which is always chat.completion.chunk.

llms/mlx_lm/SERVER.md

@@ -92,7 +92,7 @@ curl localhost:8080/v1/chat/completions \
 
 - `system_fingerprint`: A unique identifier for the system.
 
-- `object`: Any of "chat.completions", "chat.completions.chunk" (for
+- `object`: Any of "chat.completion", "chat.completion.chunk" (for
   streaming), or "text.completion".
 
 - `model`: The model repo or path (e.g. `"mlx-community/Llama-3.2-3B-Instruct-4bit"`).

llms/mlx_lm/server.py

@@ -589,9 +589,7 @@ class APIHandler(BaseHTTPRequestHandler):
 
         # Determine response type
         self.request_id = f"chatcmpl-{uuid.uuid4()}"
-        self.object_type = (
-            "chat.completions.chunk" if self.stream else "chat.completions"
-        )
+        self.object_type = "chat.completion.chunk" if self.stream else "chat.completion"
         if (
             hasattr(self.tokenizer, "apply_chat_template")
             and self.tokenizer.chat_template